A nickel-hydrogen rechargeable battery with an anode containing a hydrogen storage alloy has a higher energy density than a nickel-cadmium rechargeable battery, and places only a small burden on the environment due to absence of toxic Cd. Nickel-hydrogen rechargeable batteries are used in portable devices such as digital cameras and electric tools as well as in electric vehicles and hybrid electric vehicles. Various battery characteristics are desired depending on their applications.
As an anode material for a nickel-hydrogen rechargeable battery, there have been used a LaNi5 type hydrogen storage alloy, which is a rare earth-Ni intermetallic compound having CaCu5 crystal structure as the main phase, and a hydrogen storage alloy containing Ti, Zr, V, and Ni as constituent elements and having a Laves crystal structure as the main phase.
Rare earth-Mg—Ni based hydrogen storage alloys have recently been put into practical use. Nickel-hydrogen rechargeable batteries employing this alloy as an anode material are known to have a large capacity.
Patent Publication 1 reports that surface acid treatment of a hydrogen storage alloy containing rare earths, nickel, magnesium, and aluminum provides a hydrogen storage alloy having a higher surface content of each of the nickel, magnesium, and aluminum elements with respect to the total rare earths compared to the corresponding content inside the alloy. The publication discloses that a nickel-hydrogen rechargeable battery employing such a hydrogen storage alloy in its anode has high rate discharge performance at low temperatures, and is inhibited from decrepitation due to alloy cracking resulting from charging/discharging, to prevent deterioration of the cycle life.
Patent Publication 2 discloses that charge/discharge reaction in an alkaline aqueous solution of a nickel-hydrogen battery fabricated with an electrode containing La—Mg—Ni based hydrogen storage alloy particles and Sn, results in Sn-containing and Mg-containing compounds precipitated on the alloy particle surfaces, which increases the discharge capacity and improves the cycle characteristics of the nickel-hydrogen battery.    Patent Publication 1: JP-2007-63597-A    Patent Publication 2: JP-2009-64698-A